Mendelian Genetics

Introduction to Mendelian Genetics

Mendelian genetics is the study of how traits are inherited from one generation to the next, based on the pioneering work of Gregor Mendel. Mendel's experiments with pea plants established the fundamental laws of inheritance, which remain central to modern genetics.

Key Vocabulary

• Character: A heritable feature that varies among individuals (e.g., flower color).

• Trait: Each variant for a character (e.g., purple or white flowers).

• Gene: A heritable factor that determines a character.

• Allele: Alternative versions of a gene found at the same locus on homologous chromosomes.

• Homozygous: Having two identical alleles for a gene (e.g., PP or pp).

• Heterozygous: Having two different alleles for a gene (e.g., Pp).

• Phenotype: The observable physical or physiological traits of an organism.

• Genotype: The genetic makeup of an organism.

Mendel's Experimental System

Mendel used garden peas (Pisum sativum) for his genetic studies due to their short generation time, large number of offspring, and the ability to control mating.

• Peas have distinct characters with two contrasting traits (e.g., purple vs. white flowers).

• Mendel started with true-breeding varieties, which produce offspring identical to themselves when self-pollinated.

Mendel's Experimental Design

Mendel performed hybridization by crossing two contrasting, true-breeding varieties (P generation). The hybrid offspring (F1 generation) were then self- or cross-pollinated to produce the F2 generation.

Results of Mendel's Crosses

Mendel observed a consistent 3:1 ratio of dominant to recessive traits in the F2 generation for several characters.

Mendel's Model and the Law of Segregation

Mendel developed a model to explain the inheritance patterns he observed:

• First: Alternative versions of genes (alleles) account for variations in inherited characters.

• Second: Each organism inherits two alleles for each gene, one from each parent.

• Third: If the alleles differ, the dominant allele determines the phenotype, while the recessive allele has no noticeable effect.

• Fourth (Law of Segregation): The two alleles for a heritable character segregate during gamete formation and end up in different gametes.

Punnett Squares and Genetic Ratios

Punnett squares are used to predict the possible combinations of alleles in offspring. For a monohybrid cross (Pp x Pp), the expected genotypic ratio is 1:2:1 and the phenotypic ratio is 3:1 (dominant:recessive).

The Testcross

A testcross is used to determine the genotype of an individual with a dominant phenotype by crossing it with a homozygous recessive individual. The appearance of recessive offspring indicates the individual is heterozygous.

The Law of Independent Assortment

Mendel's law of independent assortment states that each pair of alleles segregates independently during gamete formation. This law applies to genes on different chromosomes or those far apart on the same chromosome.

Probability in Genetics

Probability rules are used to predict the outcomes of genetic crosses:

• Multiplication Rule: The probability of two independent events occurring together is the product of their individual probabilities.

• Addition Rule: The probability of any one of two or more mutually exclusive events is the sum of their individual probabilities.

For example, the probability of producing a YYRR offspring from a dihybrid cross is:

Extensions of Mendelian Genetics

Inheritance patterns can be more complex than simple dominance and recessiveness.

Degrees of Dominance

• Complete dominance: Heterozygote and dominant homozygote are phenotypically identical.

• Incomplete dominance: Heterozygote phenotype is intermediate between the two homozygotes.

• Codominance: Both alleles are fully expressed in the phenotype.

Multiple Alleles

Some genes have more than two alleles in the population. The ABO blood group in humans is determined by three alleles: IA, IB, and i.

Pleiotropy

Pleiotropy occurs when one gene influences multiple phenotypic traits. For example, the gene responsible for sickle-cell disease affects multiple organs and functions.

Epistasis

Epistasis is when the expression of one gene is affected by another gene. For example, in Labrador retrievers, one gene determines pigment color and another gene determines whether pigment is deposited in the fur.

Polygenic Inheritance

Polygenic inheritance occurs when multiple genes contribute to a single trait, resulting in continuous variation (e.g., human skin color, height).

Environmental Impact on Phenotype

Phenotype can be influenced by both genetic and environmental factors. Traits affected by multiple genes and environmental factors are called multifactorial traits.

Human Genetics and Pedigree Analysis

Human geneticists use pedigrees to study inheritance patterns in families. Pedigrees can help predict the probability of inheriting certain traits or disorders.

Recessively Inherited Disorders

Recessive disorders only appear in individuals who are homozygous for the recessive allele. Heterozygotes are carriers but do not show symptoms. Albinism is an example of a recessive disorder.

Dominantly Inherited Disorders

Dominant disorders are expressed in individuals with at least one dominant allele. Achondroplasia (a form of dwarfism) is an example. Dominant lethal alleles are rare because affected individuals often die before reproducing.

Multifactorial Disorders

Many common diseases, such as heart disease and cancer, are influenced by both genetic and environmental factors. These are called multifactorial disorders.

Genetic Counseling and Testing

Genetic counselors help individuals and families understand their genetic risks. Carrier testing and prenatal diagnosis can provide information about the likelihood of inherited disorders.

Summary Table: Mendelian vs. Non-Mendelian Inheritance